Variable rotor speed in wind turbines is desirable for several reasons. For a given rotor, the optimum rotor speed for maximum aerodynamic efficiency is a function of the wind speed. Generally, the optimum rotor speed increases with increasing wind speed. Therefore, variable rotor speed in wind turbines is desirable to maximise energy capture. Variable rotor speed also enables power generation at lower wind speeds (which correspond to lower rotor speeds), increasing the range of wind conditions that a wind turbine can operate in and thereby increasing the annual number of ‘run-hours’. Operation at lower speeds also has noise advantages because the rotor sound levels reduce when the turbine runs at lower rpm. Finally, variable rotor speed enables the torque on the transmission to be limited and smoothed at the turbine's rated power, thus reducing, to a greater or lesser extent depending on where in the transmission and how the torque is limited, the torque duty of the gearbox and transmission in general.
Since the 1990s, most commercial wind turbines have adopted some means to enable variable speed of the wind turbine rotor. Such means may be electrical or mechanical ‘variable speed’ systems. Wind turbines prior to the late 1990s typically used a conventional asynchronous induction generator with a fixed-ratio gearbox which allowed only small rotor speed variations.
Electrical variable speed systems typically enable the generator speed to vary significantly and thereby allow variable rotor speed with a fixed ratio transmission. Power electronic rectifiers and inverters are generally necessary to enable the wind turbine generator (which may be of synchronous or asynchronous design but in either case is not synchronised with the grid in the manner of a conventional synchronous generator) to be electrically connected to an alternating current electrical grid of constant frequency. This is a high cost approach as power electronic rectifiers and inverters at the rating of utility sized commercial wind turbines are expensive.
Wind turbines having electrical variable speed systems also undergo greater drive-train torque fluctuations than those having mechanical systems because the inertia of the generator rotor needs to be accelerated during gusts, giving rise to non-trivial fluctuations in the drive-train torque even though the electrical system may control the magnetic torque at the rotor-stator interface very effectively. Excessive drive-train torque fluctuations can be damaging to the wind turbine transmission and this damage mode is a continuing problem in the wind energy industry.
Mechanical variable speed systems enable the use of a directly grid connected synchronous generator, which runs at constant speed set by and synchronised with the grid. The use of a directly grid connected synchronous generator has cost advantages over electrical variable speed systems because a low cost ‘utility grade’ synchronous generator can be sourced and there is no need for expensive power electronic rectifiers and inverters. However, a directly grid connected synchronous generator must operate at a constant speed dictated by the electrical grid frequency and the number of poles of the synchronous generator. Therefore, either the wind turbine rotor speed must be constant for a fixed ratio transmission, or a variable ratio transmission must be employed to enable variable rotor speed to maximise the energy capture. Rotors in wind turbines undergo turbulence-induced torque fluctuations. Since grid-connected synchronous generators have no compliance to absorb such torque fluctuations, this means that constant rotor speed is not a practical option for wind turbines and the fluctuations must be smoothed by some other form of compliance (provided by a mechanical variable speed system) to prevent damage to the wind turbine transmission.
Mechanical variable speed systems can manage turbulence-induced torque fluctuations in wind turbines and similar power generation sources such as tidal stream generators.
Most attempts to enable variable rotor speed in wind turbines with constant speed synchronous generators have been variations of the invention described in WO 81/01444. In that document, a variable ratio transmission is achieved using one or more epicyclic gear stages connecting the main mechanical power transmission path between the wind turbine rotor and generator, to a second, parallel bypass power transmission path. The bypass power transmission path may be hydraulic or electrical with two hydraulic or electrical machines, one or both of which may be variable speed, and both of which may be variable power. The two hydraulic or electrical machines are interconnected by a means of transmitting power which may be a closed loop hydraulic or electrical circuit, as appropriate. At low rotor speed bypass power is transmitted from the generator side to the rotor side of the variable ratio transmission to increase the transmission ratio. At high rotor speed bypass power is transmitted from the rotor side to the generator side of the variable ratio transmission to decrease the transmission ratio. This type of variable ratio transmission adds significant complexity and cost to the wind turbine drive-train.
For a typical commercial wind turbine having a variable ratio transmission as described above, the maximum bypass power required to enable variable rotor speed mode in all wind speeds up to the rated wind speed, may be 20-50% of the rated wind turbine power, depending on the variable speed range and the type of transmission. FIG. 1 shows a graph of rotor efficiency against tip speed ratio (TSR), which is the ratio of the tangential speed of the tip of the rotor blade to the actual velocity of the wind. The peak of this efficiency-TSR curve for a typical rotor is relatively broad and flat, which means the incremental gains in energy capture diminish to the point of being negligible as the TSR is moved closer to the ‘optimum TSR’, where the ‘optimum TSR’ is that which gives maximum rotor efficiency (at the maximum in FIG. 1), although the cost of the bypass power system increases the wider the speed and power range over which it is active. Therefore, the benefit to cost also diminishes the wider the speed and power range over which one attempts to maintain the TSR at optimum and a practical limit is determined depending on the cost of the particular bypass power system utilised.
The above type of variable ratio transmission may also limit excessive drive-train torque fluctuations by means of an active controller as described in WO 2004/109157 and WO 2008/149109. These active controllers change the transmission ratio in a complementary way, ideally at the same rate as the transient change in turbine speed, to maintain a constant generator speed and torque while allowing the wind turbine rotor to accelerate and absorb the transient excess kinetic energy. This method of limiting excessive drive-train torque fluctuations may not always be effective due to the response time of the active controller.
U.S. Pat. No. 5,140,170 to Geoffrey M. Henderson describes a wind turbine transmission in which damaging drive-train torque fluctuations are substantially eliminated using a variable ratio transmission with a passive hydraulic torque limiting function, and an active blade pitch control system which allows small variations in wind turbine rotor speed. In the system described in that patent, a grid connected synchronous generator is driven at a constant speed and the wind turbine rotor speed is near constant until the design rated rotor torque level is exceeded due to transient aerodynamic torque fluctuations. When the design rated rotor torque is exceeded, the transmission ratio decreases rapidly due to passive hydraulic slip, the transient excess energy being stored as kinetic energy as the wind turbine rotor accelerates, and also being dissipated as heat by the hydraulic system.
The active blade pitch control in U.S. Pat. No. 5,140,170 prevents rotor over-speed at high wind speeds when the aerodynamic rotor power available is sufficient to operate at or above the design rated rotor torque level. This torque limiting system is simple and cost effective, however the narrow band of variable rotor speed achieved (typically with up to 5% rotor speed variation above the minimum rotor speed) enables the torque limiting system but does not increase energy capture other than by recovering kinetic energy stored in the rotor during transient fluctuations in wind speed at the rated power level. Such a system is generally configured so that the optimum TSR, and hence peak aerodynamic rotor efficiency, is achieved at a certain wind speed. At lower wind speeds the rotor speed is faster than the ideal rotor speed for the optimum TSR and at higher wind speeds the rotor speed is slower than the ideal rotor speed for the optimum TSR. Having a faster than optimum rotor speed at low wind speeds also means that the wind turbine will have a relatively high cut-in wind speed and relatively high turbine sound levels in light winds.
In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents or such sources of information is not to be construed as an admission that such documents or such sources of information, in any jurisdiction, are prior art or form part of the common general knowledge in the art.
It is an object of at least preferred embodiments of the present invention to provide a simple and cost effective control system for a torque limiting variable ratio transmission to provide variable rotor speed at low rotor speeds, to enable a wind turbine with a grid connected synchronous generator to operate with continuously varying rotor speed, and thereby to reduce cut-in wind speed, reduce turbine sound levels in light winds, and increase energy capture across a broader wind speed range compared to a near constant rotor speed wind turbine, and/or to provide the public with a useful alternative to electrical variable speed systems.